The display on a cathode-ray oscilloscope (c.r.o.) shows a sound wave, as illustrated. The c.r.o. time-base is adjusted to $2.5\,\text{ms cm}^{-1}$. What is the frequency of this sound wave?
- A$50\,\text{Hz}$
- B$100\,\text{Hz}$
- C$200\,\text{Hz}$
- D$400\,\text{Hz}$